(Pyrrophyta) A division consisting predominantly of flagellate unicellular marine *algae
, often dark brown in colour due to the presence of the xanthophyll pigment peridinin. It is divided into two classes, the Desmophyceae and the Dinophyceae, or dinoflagellates. The Dinophyceae are abundant in the phytoplankton. They characteristically have one longitudinal and one transverse groove in the cell wall and two flagella, one situated in each groove. The Desmophyceae either lack a cell wall or have a wall divided into two semicircular halves by a transverse groove. Certain tropical members of the Desmophyceae exhibit bioluminescence.